I guess just make sure the old files themselves are actually readable by the web server. It has been known for file permissions to change during the copy phase and although the parent folder might be the same, somehow those files are set to be unreadable for some reason.
Seems I'm having a similar issue, same error message.
I've confirmed the attachments do exist in /data, yet I would not know how to do so in /internal_data/attachments - how can I read the .data files?
Anyways, I also gave the entire root directory and forward to these attachments www-data rights, and even set the attachments themselves to 755.
Any ideas? I believe this is also related to user avatars... as an admin, only mine is visible, yet it appears that no one else's avatar is set. I logged in as a test user, changed my avatar and it stuck. But when I went back to admin, that avatar for the test user was not set, which I find very weird...
You just need to check that an internal_data/ record exists at the correct place. The data is just what's uploaded -- you can rename the extension. This error generally indicates that the file doesn't exist or can't be read.
What changed from when it worked to when it didn't? Figuring that out and either undoing it or figuring out the significance is the key to this.